Spinel

A gemstone that was confused with ruby and sapphire for over 1000 years.

Red and blue spinel: Spinel occurs in a wide variety of colors. The bright reds and deep blues are spectacular specimens. It is easy to understand how early gem traders confused spinel with ruby and sapphire for over 1000 years. Specimens and photos by Arkenstone / www.iRocks.com.

Faceted spinel: Several pretty facet-cut spinels. It is easy to see how spinel can be confused with ruby and sapphire or used as an alternative stone. These spinels are about 4 1/2 millimeters in size and weigh a little less than 1/2 carat each. The top three red and pink stones were cut from material mined in Myanmar. Deep red spinel is rarer than ruby but sells at a fraction of the price. The blue stones below them were cut from material mined in Tanzania.

The Ruby and Sapphire Impostor

Spinel is a gemstone mineral that has been confused with ruby and sapphire for over 1000 years. Several of the most spectacular spinels ever discovered have been mounted in "crown jewels" and other "jewelry of significance" under the assumption that they were rubies or sapphires.

Spinel occurs in the same bright red and blue colors as rubies and sapphires. Spinel forms in the same rock units, under the same geological conditions and is found in the same gravels.
It is not surprising that ancient gem traders thought that these colorful spinels were rubies and sapphires.

Physical Properties of Spinel

Chemical Classification

Oxide

Color

Colorless, pink, red, orange, blue, purple, brown, black

Streak

Colorless (harder than the streak plate)

Luster

Vitreous

Diaphaneity

Transparent to translucent

Cleavage

None

Mohs Hardness

7.5 to 8

Specific Gravity

3.5 to 4.1

Diagnostic Properties

Hardness, octahedral crystals, vitreous luster

Chemical Composition

MgAl2O4

Crystal System

Isometric

Uses

The only significant use is as a gemstone.

Why the Confusion?

Two thousand years ago, gemstone traders did not know that spinel and corundum (the mineral of ruby and sapphire) have different chemical compositions and different crystal structures. Instead, gem traders thought that every bright red gemstone was a "ruby" and every deep blue gemstone was a "sapphire." As a result, lots of spinels are now in very important jewelry collections based on their incorrect identification as a ruby.

The Black Prince's Ruby

The most famous example of a spinel being identified as a ruby is a 170-carat bright red spinel named "The Black Prince's Ruby." The first known owner of this beautiful stone was Abu Sa'id, the Moorish Prince of Granada, in the 14th century. The stone passed through several owners and eventually made its way into the Imperial State Crown of the United Kingdom,
where it is mounted immediately above the famous Cullinan II diamond. [1, photo]

The Timur Ruby

The "Timur Ruby" is a 352.5-carat bright red spinel that is currently in a necklace of The Royal Collection that was made for Queen Victoria in 1853. The stone was found in Afghanistan and is inscribed with the names and dates of its owners back to 1612. It was part of a group of spinels from the Lahore Treasure presented to Queen Victoria by the East India Company in 1849. [2]

Diagnostic Properties: Spinel / Corundum

Property

Spinel

Corundum

Composition

MgAl2O4

Al2O3

Crystal System

Isometric

Hexagonal

Typical Crystal Form

Octahedrons, dodecahedrons

Hexagonal prisms

Hardness

7.5 to 8

9

Diagnostic Differences (Spinel, Ruby, Sapphire)

Today gemologists understand that there are significant differences between spinel and corundum (the mineral of ruby and sapphire).
The diagnostic differences are summarized in the chart on this page. Optical properties can also be used to distinguish spinel from corundum.

Gem traders in Myanmar were the first to recognize spinel as being distinctly different from ruby in the late 1500s. [3]
In Europe, spinel continued to be misidentified as ruby until the mid-1800s.

Alluvial spinel: Much of the spinel used to produce gemstones is obtained from alluvial deposits. These deposits are worked with minimal mechanization in many parts of the world. Workers wash stream sediment and visually search through the coarse sand to fine gravel fraction, looking for colorful mineral grains that might be of value. The photo above shows some alluvial spinel produced in Vietnam. Some of the particles are thoroughly worn into rounded pebbles. Others have experienced so little transport that they still have sharp crystal edges and unworn faces.

What is Spinel?

Spinel is an oxide mineral with a composition of MgAl2O4. It is very hard (7.5 to 8 on the Mohs
Hardness Scale) and is often found in octahedral crystals. It is typically found in three geologic situations: 1) as
crystals in limestones and dolomites that have been subjected to contact metamorphism; 2) irregularly-shaped grains in
basic igneous rocks; and, 3) as water-worn pebbles in alluvial deposits.

Spinel is very resistant to chemical and physical weathering. It often occurs in marble, which is much less resistant
to weathering. Spinel easily weathers out of the marble and is transported by streams. This places spinel in alluvial
deposits which are often worked for gemstones. Most of the spinels of a "ruby-red color" are produced from alluvial deposits
in Sri Lanka, Thailand, Cambodia, Vietnam, Myanmar, and other countries. Other countries where spinel
is mined include: Afghanistan, Nepal, Tajikistan, Australia, Madagascar, Nigeria, and Tanzania.

Crown of Catherine the Great: The Great Imperial Crown was made for Empress Catherine II the Great's Coronation in 1762. The large red stone at the crest of the crown is the second-largest known spinel, weighing 398 carats. It has been titled: "Catherine the Great's Ruby." [5] Creative Commons image by Hugo Gerard Ströhl.

Samarian Spinel: Photo of the Samarian Spinel, the largest known spinel in the world and part of the Iranian Crown Jewels. It weighs approximately 500 carats. It bears an inscription dating to the mid-1600s attributing its ownership to Jehangir, the Mogul Emperor of India. It was taken from India in the early 1700s, during the Afsharid Conquest. [4] Public domain image.

Uses of Spinel

The only significant use of spinel is as a gemstone. It occurs in a variety of colors (colorless, pink, red, orange, blue, purple, brown, black).
The colors that imitate ruby and sapphire are the most popular, along with an orange-red color known as "flame spinel."

Gem-quality red and blue spinels are very rare. They are much less abundant than rubies and sapphires of similar quality and color. Even with
equivalent beauty and greater rarity, their prices are much lower than ruby and sapphire. This is an example of how rarity has not determined the price.
Spinel is not as valuable because it is not as popular. Spinel has not been strongly promoted by the gem and jewelry trade because its
supply is limited and unreliable.

Occasionally an exceptional spinel or a jewelry item of historical significance is sold at auction for a very high price. One necklace containing eleven bright red spinels, totaling 1,132 carats and inscribed by Mughal emperors, sold for over $5 million. [6, photo]

The best way to learn about minerals is to study with a collection of small specimens that you can handle, examine, and observe their properties. Inexpensive mineral collections are available in the Geology.com Store.

Synthetic Spinel

The first synthetic spinel was produced in 1847 by Jacques-Joseph Ebelmen, a French Chemist. [7] Commercial production of synthetic spinels was
very limited in the 1800s. However, in the 1930s synthetic spinels in a wide variety of colors were produced to
imitate popular gemstones such as aquamarine, zircon, tourmaline, emerald, chrysoberyl, and ruby. The colors were produced by introducing metals in trace amounts into the stone by the addition of:
cobalt oxide (blue), manganese (yellow), chromium oxide (green), and iron (pink). Careful chemical procedures allowed the manufacturers to control
the color of the stones. [8]

These colored synthetic spinels were given trade names such as "Tourmaline Green," mounted in inexpensive settings and sold as "birthstone" jewelry. [8] These
synthetic stones were the first encounter with spinel for most of the consumers who purchased them.

In addition to its use as a gemstone, synthetic spinel is also used as a refractory. It is used to produce heat-resistant coatings on metal tools and as an additive in making
refractory bricks and ceramics.

Information Sources

[1] The Black Prince's Ruby, featured in an article titled: The Imperial State Crown on The Royal Collection Trust website, last accessed July 2016.

[2] The 'Timur Ruby' Necklace, an article on The Royal Collection Trust website, last accessed January 2013.

The Spinel Mineral Group

The name "spinel" is also used for a broad group of minerals with a general chemical composition of XY2O4. In this
formula "X" could be filled by: Mg, Fe+2, Zn, Mn+2, Ni, Co, or Cu. "Y" could be filled by: Al, Fe+3, Cr, V+3, Ti+4, Ge, or Sb.

Examples of spinel group minerals include gahnite, magnetite, franklinite, chromite, chrysoberyl, and Columbite-Tantalite, as shown in the chart below.